Recent requirements for higher functions and reduction in a weight and a size in electronics have accelerated high-density integration and high-density mounting of electronic components. There is, however, a problem that a highly multilayered structure associated with high-density integration and mounting leads to increase an overall thickness of a multilayer printed wiring board. Thus, there has been devised a technique that a thinner glass cloth or resin layer is employed to make a core substrate thinner.
A core base material of a common semiconductor package is a laminated body including a plated material prepared by impregnating a fibrous base material (for example, a glass cloth) with a resin and semi-curing (so-called prepreg). Currently-used core materials predominantly have a thickness of about 0.8 mm. Recently, a thickness of a core substrate has been increasingly reduced because of requirement for reduction in a weight and a size, cost reduction for a substrate such as cost reduction for members and processing and improvement in electric properties. Recently, there have been developed semiconductor packages with a core material having a thickness of about 0.4 mm and 0.2 mm or less and furthermore without a core.
However, there have been sometimes restrictions to a circuit wiring pattern due to, for example, warpage generating in a core substrate. Approaches for solving such a problem have been disclosed in, for example, the following references. Patent Reference 1 has disclosed that warpage and torsion in a prepreg can be reduced by controlling a horizontal to vertical ratio of tensile strength of an unwoven glass fabric within a predetermined range. Patent Reference 2 relates to a process for manufacturing a laminated plate for a printed circuit consisting of a surface layer and intermediate layer with a reduced warpage. In such a process, horizontal/vertical balance is optimized by controlling difference between the numbers of fibers in a woven glass fabric used for a surface layer and a strength ratio of an unwoven glass fabric used for an intermediate layer.
Patent Reference 1: Japanese Published Unexamined Application No. 1987-292428;
Patent Reference 2: Japanese Published Unexamined Application No. 1992-259543.